Actually, to first order Isp is independent of molecular weight, for a given mass flow rate and combustion power. You can show this with the equations of momentum and energy: F = mdot*v_exh, eta*P = 0.5*mdot*v_exh^2. Notice how molecular weight doesn't show up?
The stoichiometric mixture ratio for hydrolox is O/F = 8. Hydrolox engines always run fuel-rich.
Its my understanding that the exhaust velocity is inversely proportional to the average molecular wt of the gases. Perhaps that is where the MW enters your equations (hidden in the Ve term).
In any case, low mol wt is desirable. My query was regarding variation in the mixture ratio. Is there any benefit in continuous variation?I expect its complex to implement, but would there be any theoretical basis for such a practice?
OK. What actual effect does changing the ratio make?If I were to change from (say) 5:1 to 6:1 to 7:1, what effect would this have on the operation of the motor?How would the Isp change?
I assume temp would rise as the stoichometric ratio approaches.
BTW: Thank you for your patience and your responses
And, of course, the farther you get from the stoichiometric ratio, the more bulky liquid hydrogen you have to pump and the harder it is to get high thrust, which I suspect was a consideration in the design of the SSME...
And more importantly, the more massive and draggy your external tank becomes. The SSME mixture ratio was chosen with optimization of vehicle performance, not just engine performance, in mind.
Now if there was a requirement for an engine for a SSTO; would it be worthwhile making the mixture ratio vary from launch to MECO? Would there be enough potential improvement (of whatever metric) to justify the complexity?I guess max thrust at launch and max Isp at altitude. From your figures it may vary around 10 secs over the available ranges at a given altitude. Does this variation change the thrust by enough to be worthwhile? I take it Turbopump power comes into it as well, which is something I knew nothing of before now!